1. Field of the Invention
The present invention relates to a pressure activated switching device for closing or opening an electric circuit, and particularly to a safety edge for opening or stopping the movement of a door in response to contact with an object in its path.
2. Background of the Art
Pressure activated electrical switches are known in the art. Typically, such switches are used as floor mats to open or close electrical circuits. For example, floor mat switches may be placed in the vicinity of machinery to halt its operation if anyone is in dangerous proximity to the machinery. Another use for pressure activated switching devices is as safety edges for doors. Motorized doors, (for example, in garages, factories, aircraft hangars, trains, elevators, etc.) pose a hazard to persons who may be in the path of the door as it is closing. Accordingly, such doors are typically fitted with force sensing switches along their leading edges. When the door contacts an object in its path the switch closes in response to the contact pressure. Closure of the switch can be used to send a signal to the door controller to stop or reverse the motion of the door.
Various types of force sensing switches, or "sensing edges" are known. Typically such switches include electrified conductive strips separated by a void space and/or a resilient standoff (e.g. polymeric foam). When pressure is applied to the switch, as for example when it contacts an object in the path of the moving door, the conductive strips are compressed toward each other and make contact, thereby closing an electric circuit.
For example, U.S. Pat. No. 4,396,814 to Miller discloses a safety edge switching device for a door wherein a resiliently compressible structure is enclosed in a flexible, impervious sheet covering, and the interior compartment is airtight, forming a pressurized cell. The device employs a foam layer of intermittent regularly spaced grids which expose the faces of upper and lower conductive strips. The grids are defined by two parallel portions of the foam connected by a plurality of crosspieces extending laterally from one side portion to the other, thereby forming a ladder-like pattern with spaces which are not interconnected. Upon compression, upper and lower conductive strips make electrical contact with each other through the one or more spaces in the foam layer.
Other sensing edges for doors are disclosed, for example, in U.S. Pat. Nos. 5,832,665, 5,728,984, 5,693,921, 5,426,293, 5,418,342, 5,345,671, 5,327,680, 5,299,387, 5,265,324, 5,262,603, 5,260,529, 5,225,640, 5,148,911, 5,089,672, 5,072,079, 5,066,835, 5,027,552, 5,023,411, 4,972,054, 4,954,673, 4,920,241, 4,908,483, 4,785,143, 4,620,072, 4,487,648, 4,349,710, 4,273,974, 4,051,336, 3,896,590, 3,855,733, 3,462,885, 3,321,592, 3,315,050, and 3,133,167.
While the known sensing edges have performed a useful function, there yet remains a need for a simply constructed, sensitive, but durable sensing edge for a door.